1. Field of the Invention
The present invention relates to a grain-oriented silicon steel sheet suitable for use as the iron core of transformers and other electric machines, and also to a process for producing the same. The silicon steel sheet possesses both good coating properties and good magnetic properties.
2. Description of the Related Art
Grain-oriented silicon steel sheets are used mainly as a material of the iron core of transformers and rotating machines. They are required to have such magnetic properties as high magnetic flux density, low iron loss, and small magnetostriction. Nowadays, there is an increasing demand for grain-oriented silicon steel sheets superior in magnetic properties from the standpoint of energy saving and material saving.
In the production of grain-oriented silicon steel sheets superior in magnetic properties, it is important that the resulting product has a structure such that the grains of secondary recrystallization are densely arranged along the (110)[001] orientation or so-called Goss orientation.
Grain-oriented steel sheets as mentioned above are produced by the following steps. First, grain-oriented silicon steel slabs are produced which contain MnS, MnSe, AlN, BN, or the like as an inhibitor necessary for secondary recrystallization. After heating, they undergo hot rolling. The resulting hot-rolled sheets undergo annealing, if necessary, and then undergo cold rolling (down to the final thickness) once or twice or more, with any intermediate annealing interposed. The cold-rolled sheets undergo decarburization annealing. With an annealing separator (composed mainly of MgO) coated, the steel sheets undergo final finishing annealing.
The grain-oriented silicon steel sheets obtained in this manner usually have their surfaces coated with an insulating film composed mainly of forsterite (Mg.sub.2 SiO.sub.4) (which is simply referred to as "forsterite coating" hereinafter). This forsterite coating gives the steel sheets not only surface electrical insulation but also tensile stress resulting from low thermal expansion. Therefore, it improves iron loss as well as magnetostriction.
After final finishing annealing, grain-oriented silicon steel sheets are usually given a vitreous insulating coating (simply referred to as glass coating hereinafter) on the forsterite coating. This glass coating is very thin and transparent. Therefore, it is forsterite coating rather than glass coating that eventually determines the external appearance of the product. In other words, the appearance of forsterite coating greatly affects the product value. For example, any product would be regarded as inadequate if it had forsterite coating formed such that the base metal is partly exposed. Thus, the properties of forsterite coating seriously affect the product yields. That is, forsterite coating is required to have an uniform appearance without flaws, and with good adhesion to prevent peeling at the time of shearing, punching, and bending. Moreover, forsterite coating is required to have a smooth surface because the steel sheets laminated to form the iron core need to have a high space factor.
There have been disclosed various technologies to improve the magnetic properties of grain-oriented silicon steel sheets. One of them involves the use of an auxiliary inhibitor that makes up for the function of the main inhibitor such as MnS, MnSe, AlN, and BN. Among the known elements which function as auxiliary inhibitors are Sb, Cu, Sn, Ge, Ni, P, Nb, V, Mo, Cr, Bi, As, and Pb. Of these elements, Bi is known to give a much higher magnetic flux density than before (For example, Japanese Patent Publication Nos. 32412/1979 and 38652/1981, Japanese Patent Re-publication No. 814445/1990, Japanese Patent Laid-open Nos. 88173/1994 and 253816/1996). However, adding Bi to steel presents difficulties in producing good forsterite coating at the time of finishing annealing. Products with poor coating are usually rejected.
Forsterite coating is formed at the time of final finishing annealing. The formation of forsterite coating affects the decomposition of inhibitors (such as MnS, MsSe, and AlN) in steel. In other words, it also affects the secondary recrystallization which is an essential step to obtain good magnetic properties. In addition, forsterite coating absorbs the components of inhibitor which become unnecessary after the completion of secondary recrystallization, thereby purifying steel. This purification also contributes to improvement in the magnetic properties of steel sheets.
Consequently, forming a uniform forsterite coating by controlled steps is very important to obtain grain-oriented steel sheets with good magnetic properties.
Forsterite coating is usually formed by the following steps. First, a grain-oriented silicon steel sheet which has been cold-rolled to a desired final thickness is annealed in wet hydrogen atmosphere at 700-900.degree. C. This annealing is called decarburization annealing. It has the following functions.
(1) To subject the texture (after cold rolling) to the primary recrystallization so that the secondary recrystallization takes place adequately in the final finishing annealing. PA0 (2) To reduce the content of C in cold-rolled steel sheets from about 0.01-0.10 wt % to about 0.003 wt % or less so as to protect the magnetic properties of the product from aging deterioration. PA0 (3) To cause subscale (containing SiO.sub.2) to form in the surface layers of steel sheets by oxidation of Si that is present in steel. PA0 Japanese Patent Laid-open No. 185725/1984, controlling the oxygen content in steel sheets after decarburization annealing. PA0 Japanese Patent Publication No. 1575/1982, keeping the degree of oxidation in the atmosphere at 0.15 and above in the front region of decarburization annealing and at 0.75 and below in the rear region that follows. PA0 Japanese Patent Laid-open No. 240215/1990 and Japanese Patent Publication No. 14686/1979, performing heat-treatment at 850-1050.degree. C. in a non-oxidizing atmosphere after decarburization annealing. PA0 Japanese Patent Publication No. 57167/1991, cooling after decarburization annealing in such a way that the degree of oxidation is lower than 0.008 in the temperature region below 750.degree. C. PA0 Japanese Patent Laid-open No. 336616/1994, performing heat treatment in such a way that the ratio of the partial pressure of water vapor to the partial pressure of hydrogen is lower than 0.70 in soaking step and the ratio of the partial pressure of water vapor to the partial pressure of hydrogen in the heating step is lower than that in the soaking step. PA0 Japanese Patent Laid-open No. 278668/1995, prescribing the rate of heating and the atmosphere of annealing. PA0 Japanese Patent Laid-open No. 232019/1996, adjusting the amount of oxygen in oxide film after decarburization annealing to 600-900 ppm and applying an annealing separator incorporated with 0.01-0.10 pbw of chlorine compound (as Cl) and/or 0.05-2.0 pbw of one kind or more than one kind of Bb, B, Sr, and Ba compounds, for 100 pbw of MgO. PA0 Japanese Patent Laid-open No. 258319/1996, adjusting the amount of annealing separator (composed mainly of MgO) to 5 g/m.sup.2 or above on one side of steel sheet. PA0 Japanese Patent Laid-open No. 111346/1997, adjusting the flow rate of atmosphere gas in finishing annealing such that the ratio of flow rate to the total surface area of steel strip is equal to or larger than 0.002 (Nm.sup.3 /hm.multidot..sup.2). PA0 Japanese Patent Laid-open No. 25516/1998, adjusting the Ig-loss value of magnesia in the annealing separator to 0.4-1.5 wt %. PA0 Japanese Patent Laid-open No. 152725/1998, adjusting the amount of oxygen on the surface of steel sheet after decarburization annealing to 550-850 ppm.
After decarburization annealing, the steel sheet is coated with an annealing separator (composed mainly of MgO) and then coiled. The coil undergoes final finishing annealing (which serves also for secondary recrystallization and purification) in a reducing or non-oxidizing atmosphere at about 1200.degree. C. (maximum). Forsterite coating is formed on the surface of steel sheet according to the solid-phase reaction shown by the following formula. EQU 2MgO +SiO.sub.2.fwdarw.Mg.sub.2 SiO.sub.4
Forsterite coating is a ceramic coating densely composed of fine crystalline particles about 1 .mu.m in size. As the formula shows, one raw material of forsterite coating is subscale containing SiO.sub.2 which has formed in the outer layer of the steel sheet at the time of decarburization annealing. Therefore, the kind, amount, and distribution of subscale are deeply associated with the nucleation and grain growth of forsterite coating. They also greatly affect the strength of grain boundary and grain of coating crystals and further affect the quality of coating after final finishing annealing.
The annealing separator (composed mainly of MgO as another raw material) is applied to the steel sheet in the form of an aqueous slurry. Therefore, steel sheets retain physically adsorbed water even after drying, and MgO partly hydrates to form Mg(OH).sub.2. As the result, steel sheets continue to give off water (although small in quantity) until the temperature reaches about 800.degree. C. during final finishing annealing. This water oxidizes the surface of the steel sheet during final finishing annealing. The oxidation by water also affects the formation of any forsterite coating and the behavior of inhibitors. Added oxidation by water is a factor tending to deteriorate magnetic properties. In addition, the ease with which oxidation by water takes place depends greatly on the physical properties of subscale formed by decarburization annealing.
Also, any additives other than MgO incorporated into the annealing separator, however small in quantity, greatly affect the film formation as a matter of course.
In the case of grain-oriented silicon steel sheets having a nitride inhibitor (such as AlN and BN), the physical properties of subscale greatly affect the behavior of denitrification during finishing annealing or the behavior of nitrification from the annealing atmosphere. Therefore, the physical properties of subscale greatly affect the magnetic properties of the sheet.
As mentioned above, controlling the physical properties of subscale formed in the outer layer of steel sheets during decarburization annealing, controlling the properties of magnesia in the annealing separator, and controlling the kind of additive in the annealing separator are three factors indispensable in forming forsterite coatings of uniform good quality at a prescribed annealing temperature which is determined by the condition of secondary recrystallization in finishing annealing. They are very important in the production of grain-oriented steel sheets.
Incidentally, if the steel does not contain Bi, a forsterite coating of good quality may be formed by any of the disclosed techniques given below.
Forsterite coating looks poor if the base metal is exposed sporadically. This defect can be avoided by the method disclosed in Japanese Patent Laid-open No. 226115/1984, which consists of causing the raw material to contain 0.003-0.1 wt % of Mo and performing decarburization annealing at 820-860.degree. C. such that the degree of oxidation in the atmosphere is 0.30-0.50 expressed as P(H.sub.2 O)/P(H.sub.2), and the subscale formed on the surface of steel sheet is composed of silica (SiO.sub.2) and fayalite (Fe.sub.2 SiO.sub.4), with the ratio of Fe.sub.2 SiO.sub.4 /SiO.sub.2 being in the range of 0.05-0.45.
Apart from the above-mentioned techniques relating to decarburization annealing, there have been proposed a number of techniques for improving the characteristic properties of the coating film. These techniques involve the addition of a Ti compound (such as TiO.sub.2), as an additive other than magnesia, to the annealing separator. For example, Japanese Patent Publication No. 12451/1976 discloses a method of improving the uniformity and adhesion of a forsterite coating by incorporating 100 pbw of Mg compound with 2-40 pbw of Ti compound. Japanese Patent Publication No. 15466/1981 discloses a method of eliminating black spots from the Ti compound by finely grinding TiO.sub.2 for the annealing separator. Japanese Patent Publication No. 32716/1982 discloses a method of adding an Sr compound in an amount of 0.1-10 pbw (as Sr) so as to form a forsterite insulating film with good adhesion and good uniformity.
Also, there have been disclosed several methods for improving the magnetic properties by adding a compound to the separator. Japanese Patent Publication No. 14567/1979 discloses the addition of Cu, Sn, Ni, or Co, or a compound thereof in an amount of 0.01-15 pbw (as metallic element). Japanese Patent Laid-open No. 243282/1985 discloses the addition of TiO.sub.2 or TiO (0.5-10 pbw) and SrS, SnS, or CuS (0.1-5.0 pbw), together with optional antimony nitrate (0.05-2.0 pbw).
Moreover, Japanese Patent Laid-open No. 291313/1997 discloses a method of improving both the magnetic properties and the film characteristics of the sheet. This method is based on the result of investigation on the relation between the subscale (which occurs at the time of decarburization annealing) and the annealing separator. The object is achieved by adjusting the partial pressure of hydrogen (P(H.sub.2)) and the partial pressure of water vapor (P(H.sub.2 O)) in decarburization annealing such that the ratio of P(H.sub.2 O)/P(H.sub.2) in the soaking step is lower than 0.70 and the ratio of P(H.sub.2 O)/P(H.sub.2) in the heating step is lower than that in the soaking step, and also by incorporating 100 pbw of MgO in the annealing separator with 0.5-15 pbw of TiO.sub.2, 0.1-10 pbw of SnO.sub.2, and 0.1-10 pbw of Sr compound (as Sr).
There have been proposed other techniques developed, with attention paid to the amount of subscale in steel sheets which have undergone decarburization annealing. For example, Japanese Patent Laid-open Nos. 329829/1992 and 329830/1992 disclose a method of adding Cr and Sb simultaneously or adding Cr, Sn, and Sb simultaneously, thereby minimizing the fluctuation of the amount of oxidized layer and forming the coating film stably in finishing annealing. Japanese Patent Laid-open No. 46297/1989 discloses a method of making fayalite (Fe.sub.2 SiO.sub.4) and silica (SiO.sub.2) thick enough for the formation of forsterite coating by adding Cr and establishing adequate conditions for decarburization annealing so as to promote diffusion of oxygen in the thickness direction.
Unfortunately, incorporating steel with Bi suffers difficulties in obtaining a good forsterite coating at the time of finishing annealing (which results in unacceptable products with poor coating film). In connection with this, Japanese Patent Laid-open No. 202924/1997 mentions that "it is assumed that Bi vapor concentrated between steel sheets adversely affects the formation of primary coating, thereby making it difficult to form good primary coating film." Incidentally, this Japanese Patent discloses a method of increasing the magnetic flux density by the addition of Bi and also providing a material with low iron loss. (This method is based on the above-mentioned assumption.)
Even in the case of Bi-containing steel, good forsterite coating can be obtained by any of the methods disclosed as follows.
Incidentally, the Ig-loss value is hydrate amount calculated by the weight difference between before and after baking process of making magnesia.
The above-mentioned techniques, however, do not basically change the reaction to form forsterite in the presence of Bi (or do not promote the forsterite reaction 2MgO+SiO.sub.2.fwdarw.Mg.sub.2 SiO.sub.4). In other words, they do not improve forsterite coating satisfactorily, or they cannot stably form defect-free, uniform forsterite coating of good quality and good adhesion over the entire width and length of a coil product.